With an increase in demand for communication, optical networks that use wavelength division multiplexing (WDM) are widely used. WDM is a technique for multiplexing a plurality of optical signals with different wavelengths and transmitting the optical signals. For example, WDM enables 88 optical signals to be multiplexed and transmitted as a wavelength multiplexed optical signal (hereinafter referred to as multiplexed optical signal).
As a transmission device using WDM, an optical add-drop multiplexer (OADM) that is referred to as a reconfigurable optical add-drop multiplexer (ROADM) or the like is known. The optical add-drop multiplexer has a transceiver that transmits and receives optical signals. The transceiver is referred to as a transponder or the like.
The optical add-drop multiplexer multiplexes optical signals received from a transceiver and having arbitrary wavelengths and transmits the multiplexed signals to another node. The optical add-drop multiplexer separates an optical signal with an arbitrary wavelength from multiplexed optical signals received from the other node and outputs the optical signal through the transceiver. In other words, the optical add-drop multiplexer adds and drops an optical signal with an arbitrary wavelength.
The optical add-drop multiplexer has an optical amplifier and a wavelength selective switch (WSS) for each path.
The wavelength selective switch selects a specific wavelength from among received multiplexed optical signals and outputs an optical signal with the selected wavelength from an arbitrary port. The wavelength selective switch multiplexes optical signals input from an arbitrary port and outputs the optical signals as a multiplexed optical signal.
The wavelength selective switch has a filter for a passband of which the central wavelength is the selected wavelength (frequency). When a plurality of optical add-drop multiplexers are connected to each other, the width of a passband is reduced due to accumulated errors of the filters, and the spectrums of optical signals are narrowed.
The spectral width of an optical signal depends on a scheme for modulating the optical signal. The higher the bit rate, the larger the spectral width of the optical signal. For example, the spectral width of an optical signal transmitted at a high rate of 40 Gbps, 100 Gbps, or the like is large. When the large spectral width is reduced by filtering at multiple stages, an error rate may increase. Thus, the number of optical add-drop multiplexers that transmit an optical signal while maintaining the quality of the transmission at a certain level is limited.
Regarding narrowing of a spectrum, Japanese Laid-open Patent Publication No. 2010-98544 discloses that if a path is not assigned to wavelengths that are next shorter and longer than a wavelength of a certain path, a passband of a filter for the certain path is maximized for a wavelength selective switch. transmission device